The present invention relates to a method of wood pressing in manufacturing glued laminated wood made of a number of wood sheets such as veneer sheets which are provided by rotary peeling or otherwise prepared, dried appropriately, clipped into a predetermined length, and laminated together by hot pressing with a thermosetting adhesive in such a way that a multiple-layered board of glued laminated wood is made. The invention also relates to a wood pressing apparatus for use in manufacturing the glued laminated wood.
For better understanding of the underlying background of the present invention, firstly reference is made to FIG. 1 which shows a drawing provided in the publication of “LVL Laminated-Veneer Lumber—Development and Economics” by J. C. Bohlen, Forest Prod. J. Vol. 22, No. 1, 1972. The drawing schematically shows a prior art apparatus for manufacturing LVL, that is a typical glued laminated wood, wherein a number of veneer sheets are laminated together in a staggered manner with the wood grain of each veneer sheet running primarily along the length of the resulting product or LVL board.
The apparatus includes three sets of hot presses X, Y and Z arranged at three different stages spaced at an interval along the direction in which LVL board in process is conveyed as indicated by arrow at the right-hand side on the drawing. Each of the hot presses X, Y and Z has a pair of movable upper and lower heating plates X1, Y1 and Z1 disposed one above the other, respectively. For the sake of description of the apparatus, these three sets of hot presses X, Y and Z will be referred to as the first, second and third hot presses, respectively. Though not shown specifically in the drawing, there are provided three sets of veneer feeders associated with the respective hot presses X, Y and Z, and each set includes upper and lower movable veneer sheet feeders as indicated by arrows which depict the feeding directions. In the drawing, reference symbols A, B and C designate wood veneer sheets each having substantially the same length, width and thickness and disposed in the same orientation with the wood grain thereof extending substantially parallel to each other, i.e. extending along the lengthwise direction of the LVL board in process. It is noted that, for the sake of convenience in illustration, each veneer sheet is illustrated with a thickness which is shown exaggeratedly large relative to its length and also that a conveyer for moving a board in process is omitted from the illustration.
In manufacturing LVL board with the apparatus of FIG. 1, firstly a pair of veneer sheets A depicted by shading is prepared with one surface of either one of such veneer sheets A coated with thermosetting adhesive. These veneer sheets A are fed by the first feeders for the first hot press X and combined together with the adhesive-coated surface interposed between the mating surfaces of the two veneer sheets A and with the ends thereof in an offset or staggered relation as shown in the drawing in such orientation that their wood grain extends generally along the length of LVL board to be produced. These two veneer sheets A thus placed one on the other are conveyed to the first pressing station between the first heating plates X1, where the veneer sheets A are glued together under heat and pressure by the hot press X. After the first hot pressing operation is over and the heating plates X1 are retracted, the laminated veneer sheets A are conveyed toward the second pressing station at the second hot press Y.
On the way to the second pressing station, i.e. at an appropriate position between the first and second pressing stations, another pair of veneer sheets B, indicated by shading, each having on one surface thereof adjacent to the laminated veneer sheets A coated with thermosetting adhesive is fed by the second veneer sheet feeders and laid on the opposite outer surfaces of the previously laminated veneer sheets A in a staggered relation to the veneer sheets A as shown in the drawing. The veneer sheets B thus laid onto the veneer sheets A are conveyed therewith to the second pressing station, where the veneer sheets B are pressed by the second hot press Y to be bonded to the laminated veneer sheets A.
After pressing by the second hot press Y is over, the laminated veneer sheets A and B are conveyed toward third hot press Z at the third station. At an appropriate position between the second and third pressing stations, still another pair of veneer sheets C, indicated by shading, having on the inner surfaces thereof coated with adhesive is fed by the third veneer feeders and laid on the opposite outer surfaces of the veneer sheets B in a staggered arrangement. The veneer sheets C thus placed on the laminated veneer sheets A and B are conveyed to the third pressing station, where they are similarly pressed by the third hot press Z. Thus, an assembly of laminated veneer sheets A, B and C is formed.
Steps of operation including the above veneer sheet feeding, laying, hot pressing and conveying are performed successively at the respective positions and stations, whereby an LVL board with six plies as counted through its thickness is formed. As shown in FIG. 1, any two adjacent veneer sheets A, B or C in each layer are disposed with the ends thereof abutting closely each other to form end joints, and the veneer sheets A, B and C are laid such that the end joints are distributed in a regularly staggered arrangement in the resulting LVL board.
The above-cited publication which provides the drawing of FIG. 1 showing a method and an apparatus for manufacturing LVL gives no description about the manner of operation of the respective hot presses. A hot press having a pair of upper and lower heating plates for pressing therebetween wood material such as veneer sheets for manufacturing LVL usually has one of the heating plates movable toward the other heating plate which is stationary or fixed. If such a hot press is used in the apparatus of FIG. 1, problems will occur as will be described below.
If the first, second and third hot presses X, Y and Z shown in FIG. 1 are of such structure that the lower heating plates are fixed and the upper heating plates are movable toward and away from the respective lower heating plates, the lower heating plates of the hot presses Y and Z should be arranged such that their top pressing surfaces are lower than the top pressing surface of the lower heating plate of the first hot press X by distances corresponding to the thickness and twice the thickness of veneer sheet, respectively. As apparent from the drawing of FIG. 1, however, feeding of veneer sheets B or C is extremely difficult in such arrangement of the lower heating plates.
At the hot press Y, for example, a veneer sheet B will have to be inserted into a space between the lower surface of veneer sheet A already pressed in the previous first hot press X and the top surface of the lower heating plate Y1 of the second hot press Y. Since the spaced distance between the above two surfaces of the veneer sheet A and the lower heating plate Y1 corresponds to the thickness of veneer sheet, it would be practically impossible to insert a veneer sheet B into such a limited small space and set it in place. The same is true of a veneer sheet C which is to be inserted into a similar limited small space at the third hot press Z.
If the lower heating plates Y1 and Z1 of the second and third hot presses Y and Z are set such that their top pressing surfaces are lower than the top pressing surface of the lower heating plate X1 of the first hot press by distances which are slightly greater than the thickness and twice the thickness of veneer sheet, respectively, it is still difficult to feed a veneer sheet B or C. At the second hot press Y, for example, veneer sheet B already glued to LVL in process (or the right veneer sheet B of two veneer sheets B which are shown in contact with the lower heating plate Y1 of the second hot press Y in FIG. 1) tends to be brought into contact with the lower heating plate Y1 by bending of the in-process LVL board by its own weight. As a result, the space between the lower surface of veneer sheet A already glued at the previous first hot press X and the top surface of the lower heating plate Y1 of the second hot press Y is narrowed to a distance corresponding to the veneer sheet thickness.
The spaced distance between the top surface of the lower heating plate X1 of the first hot press X and the top surfaces of the lower heating plates Y1 and Z1 of the second and third hot presses Y, Z may be set much larger so that a veneer sheet, for example veneer sheet B, is insertable even if the in-process LVL board is bent by its own weight. When the upper heating plate Y1 is moved down toward its associated lower heating plate Y1 for pressing, however, the in-process LVL board is bent down by the lowering upper heating plate Y1, so that the resulting LVL board is bent into an arcuate shape. The same holds true at the hot pressing station Z. The LVL board thus bent in arcuate shape is rendered useless as a product.
Therefore, an object of the present invention is to provide a method and an apparatus of wood pressing which can be used helpfully in manufacturing glued laminated wood such as LVL.